In some forums it could encourage lurkers to sign up (and reduce the barrier to subsequent participation) if the first post were visible to all users, but replies only visible to logged-in users. (There might be other variations based on specific groups.)
I also wonder whether it would help reduce AI scraping, but maybe that is futile…
I’m aware of Gated Topics in Category but have in mind something that doesn’t send the full page to the browser at all. And shows the full first post rather than just a percentage of the screen.
I am testing an internal fork of the Discourse Category Lockdown plugin to allow users to See category description topics with no other access (on tiered or subscription-based categories).
I think that the proposed permissions scope could be very useful and be implemented in plenty of use cases.
Most of the logic for this is already in the private replies plugin. We’d only need to change the business rules for when the replies become visible although it should already be possible with some (unlogical) settings tweaking.